ACE and UCP2 gene polymorphisms and their association with baseline and exercise-related changes in the functional performance of older adults

Introduction

Maintaining adequate levels of muscle mass, strength, muscular and aerobic endurance and functional performance in older age is important, as a decline in these physical attributes may result in: (1) a loss of independence (Baumgartner et al., 2004; Kim et al., 2012), (2) an increased risk and fear of falls (Brouwer, Musselman & Culham, 2004; Wagenaar, Keogh & Taylor, 2012); (3) an increase in the risk of chronic conditions (Abellan van Kan et al., 2009); and (4) a reduction in quality of life (Giles, Hawthorne & Crotty, 2009; Masel et al., 2009). Engaging in regular resistance and aerobic activity and ensuring adequate nutritional intake appear to be some of the key strategies for older adults to reduce these sarcopenic-related losses of muscle mass, strength and function (Fiatarone-Singh, 2002; Nelson et al., 2007). While initial studies in this area focused on maintaining muscle mass, a number of recent reviews indicate older adults at risk of physical decline may gain more substantial improvements in muscular strength, balance and functional performance, for example gait speed, than improve their muscle mass (Keogh & MacLeod, 2012; Valenzuela, 2012). This is vital as the age-related loss of strength, balance and gait speed has a greater relationship to outcomes such as activities of daily living, independence and quality of life and mortality than muscle mass alone (Abellan van Kan et al., 2009; Kim et al., 2012; Wood et al., 2011).

Studies involving younger (Argus et al., 2009; Till & Cooke, 2009) and middle-aged to older (Karavirta et al., 2011) adults indicate the potential for considerable inter-individual responses to identical training programs, even when the sample is relatively homogenous at baseline and engage in the same exercise program. For example, in a study in which 175 untrained middle-aged to older adults performed 21 weeks of strength, aerobic training, combined strength and aerobic or no training, Karavirta et al. (2011) reported large individual differences in muscular strength (−12 to 87%) and aerobic power (−8 to 42%) changes for the combined strength and endurance training group (n = 53). This high degree of within-participant variability in response was demonstrated via the use of criterion assessments of strength and aerobic power, namely the maximum voluntary contraction (MVC) and cycle ergometry peak oxygen consumption (VO_2peak) tests, respectively. Further, there were no significant correlations (all r’s < 0.10) between the changes in the MVC strength or VO_2peak in any of the three groups, suggesting that the strength and aerobic power responses to strength and/or aerobic training can exhibit substantial within- and between-participant variability. These intra-individual differences in training response may reflect genetic factors.

While considerable heterogeneity exists in the physical function of many older adults, even when chronic conditions and medications are controlled for, the relative contribution of genetics and long-term physical activity levels in determining their physical function and ultimately survival remains less clear. Based on research involving primarily younger adults, two genes that might play a role in regulating the physical function and indirectly impact on survival in older adults are angiotensin-converting enzyme (ACE) and mitochondrial uncoupling protein 2 (UCP2) (Dhamrait et al., 2012; Puthucheary et al., 2011).

When expressed, the ACE gene produces angiotensin-converting enzyme protein. This ACE protein plays a key role in regulating the activity of the rennin-angiotensin system, thereby directly influencing blood pressure and fluid balance and indirectly influencing cardiovascular and musculoskeletal structure and function (Puthucheary et al., 2011; Seripa et al., 2011). Indeed, a functional polymorphism in the human ACE gene, the intron 16 insertion (I) allele, has been associated with an enhanced cardiovascular response to training (Myerson et al., 1999), while the deletion (D) allele has been associated with a superior muscle size and strength response to training (Puthucheary et al., 2011; Woods et al., 2001).

The UCP2 gene is expressed in many tissues particularly skeletal muscle. It exerts a variety of effects on mitochondrial function, meaning it influences the rate of synthesis of ATP and reactive oxygen species and indirectly influences many indices of cardiovascular health (Bo et al., 2013; Dato et al., 2014; Dhamrait et al., 2004; Palmer et al., 2003). The frequency of the UCP2 (A55V) C allele was found to be higher among power athletes compared with controls (Sessa et al., 2011). Another allele of the UCP2 G-866A polymorphism has also been associated with increased delta efficiency after endurance training in young, healthy adults compared to those carrying the G allele (Dhamrait et al., 2012; Perusse et al., 2013).

Recently, some studies have examined the relationships between ACE (Bustamante-Ara et al., 2010; Garatachea et al., 2012; Giaccaglia et al., 2008; Pereira et al., 2013) and UCP2 (Dato et al., 2014) gene polymorphisms and a variety of measures of muscular strength and power in older adults. Only one of these four ACE studies reported significant genotypic relationship to performance at baseline, with the ACE insertion/deletion (ACE ID) group having a significantly greater baseline handgrip strength (Bustamante-Ara et al., 2010). In support of a possible genetic link to baseline function, Seripa et al. (2011) demonstrated that the ACE II genotype was associated with increased risk of limitations in activities of daily living in hospitalised older patients. Of the two studies that assessed training-related changes in older adults, both reported significantly greater improvements in strength and power measures for those with the ACE D allele (Giaccaglia et al., 2008; Pereira et al., 2013). To the authors’ knowledge, only one study has so far reported associations between baseline differences in older adults who differ in UCP2 genotypes (Dato et al., 2014), with no studies examining the exercise response. Dato et al. (2014) found significantly greater walking speeds over 3–4 m in those with the rs7109266 SNP found on the UCP2 G-866A allele, but no UCP2 association with the degree of disability, handgrip strength or cognitive function in a sample of 1089 older adults.

Therefore, the purpose of this study was to determine if either the ACE and/or UCP2 genotypes individually: (1) distinguish the baseline level of functional performance in older adults who are not currently performing an exercise program; and (2) influence the training-related changes in older adults’ functional performance. It is hypothesised that variations in these individual alleles will be associated with baseline- and training-related differences in older adults’ functional performance.

Materials and Methods

Training Program

Participants attended the N2O program twice weekly for 12 weeks, with each training session lasting approximately 60 min. The participants in this research project performed the Bronze N20 program, which focused on providing a safe and friendly environment that introduced the older adults to selected resistance and cardiovascular exercises that were aimed at improving their overall functional performance and health.

Training sessions commenced with a 5–10 min warm-up that preceded the resistance training component. The resistance training exercises included the knee extension, leg curl, leg press, chest press, lat pulldown, shoulder press, bicep curl and tricep pushdown, with a selection of these exercises performed during each training session. Each exercise was performed for 1–2 sets of 8–12 repetitions with loads that initially produced moderately light to moderate ratings of perceived on the Borg rating of perceived exertion (RPE) scale (Borg, 1982). After completing the resistance training exercises, 5–10 min of cardiovascular exercise (i.e., stationary cycling or walking on a treadmill) was then performed at a moderately light to moderate intensity on the Borg RPE scale (Borg, 1982). The exercise session was then completed by performing 5–10 min of stretches for the major muscle groups of the body. Loads for the resistance and cardiovascular exercises were progressively increased over the course of the 12 week training program.

Results

A description of the participants’ baseline demographic data is provided in Table 1. Almost all participants had at least one chronic disease, with high blood pressure (19%), arthritis and other joint conditions (16%) and cardiovascular disease (12%) being most common. At baseline, most participants were somewhat physically active (i.e., performed ∼60 min of moderate physical activity per week) and possessed moderately low to moderately high levels of physical function, based on age- and gender-matched normative data for the Senior Fitness Test (Rikli & Jones, 1999b). Program attendance was very high with the participants attending 22 ± 2 out of the requested 24 training sessions, resulting in an overall attendance rate of 93%.

DNA samples were successfully obtained from all participants who commenced the study. However, a small number of subjects did not complete all physical tests during the pre- and post-training assessments. Therefore, the actual sample size for each test differed somewhat for each of the tests, with this presented within the Tables. Importantly, the resulting genotype frequencies for each of the studied polymorphisms conformed to the Hardy-Weinberg equilibrium (Table 2).

Discussion

The current study’s primary aims were to determine if baseline differences and training-related changes in functional performance were related to particular genotypes of the ACE and UCP2 genes in older adults. The frequencies of the ACE and UCP2 G-8661 genotypes in our sample conformed to the expectations of the Hardy-Weinberg equilibrium and to that of a New Zealand population of Caucasian ancestry (Palmer et al., 2009; Palmer et al., 2003).

At baseline, it was observed that the ACE gene variant genotypes were associated with significant differences in 3 of the 6 performance tests (6 min walk, 8ft Up and Go and handgrip strength). Baseline variations in the UCP2 gene polymorphism were also observed for the 8ft Up and Go, with handgrip strength approaching significance (p = 0.094).

The ID/DD ACE allele groups performed significantly better at baseline than those with those with II alleles on the 6 min walk and 8ft Up and Go tests. While these two walking tests both require the participants to walk as quickly as possible over a set course, these tests differ in several ways. The 6 min walk test requires the participants to walk a maximum distance in 6 min, and is therefore considered a field test of VO_2peak and walking ability for older adults (Rikli & Jones, 1999a; Rikli & Jones, 1999b). In contrast, the 8ft Up and Go requires the participants to stand up, walk a distance of 8 ft, turnaround, walk back to the chair and then sit down, suggesting a greater reliance on lower body strength, walking speed and mobility (Rikli & Jones, 1999a; Rikli & Jones, 1999b).

The baseline differences for the 8ft Up and Go appear consistent with previous studies demonstrating increased muscular strength or power with the D allele of the ACE gene for younger adults (Puthucheary et al., 2011; Woods et al., 2001), but in contrast to some studies involving older adults (Garatachea et al., 2012; Giaccaglia et al., 2008; Pereira et al., 2013). While the greater 6 min walk distance for the ID/DD groups was initially unexpected, a lack of lower body strength may reduce the ability of older adults to walk briskly for extended periods of time. Research support for this view is provided by the significant improvements in VO_2peak observed in older adults as a consequence of resistance training (Lovell, Cuneo & Gass, 2009). The significantly greater performance in the two walking tests for the ID/DD groups is also consistent with a recent report (Seripa et al., 2011), observing that the ACE II genotype was associated with greater limitations in activities of daily living in hospitalised older patients. As gait speed over a variety of distances is an influential determinant of maintaining independence and in reducing many age-related adverse effects (Abellan van Kan et al., 2009), the results of our study and (Seripa et al., 2011) suggest some genetic component to the wide variation seen in walking speed and performance of older adults (Peel, Kuys & Klein, 2013).

Based on the reported role of the ACE gene in human physical performance studies involving younger adults (Dhamrait et al., 2012; Puthucheary et al., 2011), our findings at baseline for the 30 s STS and handgrip strength test were somewhat unexpected, with greater performance expected from those with the ID/DD alleles for both of these tests. A significant between-group was observed for handgrip strength, whereby the ID group had greater handgrip strength than the DD group. While such a result was not expected based on the results of the younger adult literature, it was consistent with the only other study conducted to date involving older adults (Bustamante-Ara et al., 2010). Our results indicated no significant baseline differences between ACE genotype groups for the 30 s STS. While this may have unexpected based on the younger adult literature, the lack of significant baseline differences in 30 s STS performance was also consistent with the results reported for 139 older women by Pereira et al. (2013).

Comparisons of the baseline performance results for the UCP2 genotype groups also revealed significant differences for the 8ft Up and Go test and a trend for handgrip strength (p = 0.094), whereby greater performance was found in those with the A than G UCP2 allele. This result was somewhat consistent with (Dato et al., 2014), who reported significantly greater walking speeds over 3–4 m in older adults with the UCP2 A allele. However, the direction of these baseline differences for the 8ft Up and Go test was inconsistent with the greater improvements in endurance performance markers for the A compared to G allele carriers in younger populations (Dhamrait et al., 2012; Perusse et al., 2013). Such inconsistencies in the association between UCP2 polymorphisms and endurance vs power performance in younger compared to older adults warrants further investigation.

The second aim of the current study was to examine whether the ACE and UCP2 gene influenced the training related responses in older adults’ functional performance. The training program used in the current study appeared effective as the magnitude of the training related changes for the Functional Fitness Test battery outcomes was relatively consistent with that reported for other community-based older adult exercise programs (Bates et al., 2009; Belza et al., 2010; Henwood, Wooding & de Souza, 2013). Inspection of the data revealed little evidence for the ACE or UCP2 gene alleles to have any significant effects on the training-related changes in the outcome measures assessed in this study. Exceptions to this rule were the significantly greater improvements for the GG UCP2 group in the 8ft Up and Go and the trend (p = 0.099) for larger improvements in bicep curl strength for the ID/DD versus II ACE allele groups. These associations between the UCP2 gene and training-related changes in the 8ft Up and Go test are novel and to the authors’ knowledge, our study is the first to assess the relationship between the UCP2 genotypes and changes in physical performance in older adults.

The lack of differences in the training related response between different ACE genotype groups was consistent with the only study to assess changes in older adults’ 6 min walk performance resulting from 18 months of combined aerobic and resistance training (Giaccaglia et al., 2008). The responses in our assessments that were more dependent on strength or power were inconsistent with the two studies conducted to date, where significantly greater improvements in muscular strength and power were associated with the ACE D gene allele (Giaccaglia et al., 2008; Pereira et al., 2013). Potential reasons for the variation in our results compared to the two studies within the literature may be explained by three primary factors.

The first of these was the difference in outcome measures assessed in the current study compared to that of the literature. The current study focused on functional performance tests (30 s STS, bicep curl, 8 ft Up and Go and 6 min walk), whereas most of the outcomes assessed in the literature were strength or power tests. While performance in many of the functional performance tests we selected is moderately correlated with strength and/or VO_2peak, our assessments were not criterion measures of either of these physical qualities. It is therefore likely that functional performance tests have a greater variety of determinants than criterion tests of muscular strength or VO_2peak, thereby reducing the potential for isolated genotypes to significantly influence training-related changes in functional performance. Such speculation is consistent with the lack of any significant relationship between the ACE and UCP2 genotypes for the older adults’ baseline- or training-related differences in upper limb dexterity (Purdue Pegboard) test performance seen in this study. The second difference may have been the relatively high levels of baseline function for some genotype groups in the current study compared to that of the literature. Specifically, the ACE ID/DD and UCP2 AA gene allele groups who had the highest scores at baseline, were typically in the 60–80th percentile (men) and 80–90th percentiles (women) for the 30 s STS, bicep curl, 6 min walk and 8 ft Up and Go tests (Rikli & Jones, 1999b). In comparison, our other gene allele group and older adults in the other studies within this literature appeared to have much poorer baseline function. These weaker older adults in ours and other studies therefore had more chance to demonstrate significant improvements in function and significant genotype-related responses to training than our higher functioning ID/DD and UCP2 AA groups. Thirdly, the current study had more modest statistical power for detecting training-related changes (70% power) than baseline differences (90% power).

In conclusion, the current study adds to the literature on the potential link between genotype and physical performance in older adults. While our baseline results were generally consistent with the phenotypes associated with the ACE I/D and UCP2 polymorphisms, we found no strong evidence of genetic association to the functional performance response to physical training in this study. Some of this relative equivalence may have reflected our focus on functional performance such as the 8ft Up and Go test rather than physical capacity outcomes e.g., MVC strength or aerobic power. Specifically, functional performance assessments would appear to require the integration of a greater number of sensoriomotor functions than physical capacity assessments of MVC or VO_2peak. The variance in results for this study compared to that of literature may also reflect the relatively high levels of baseline function for some genotype groups of the older adults in the current study as well as the greater statistical power for detecting baseline differences (90% power) than the training-related changes (70% power). Future studies in this area should recruit larger sample sizes so to obtain sufficient statistical power to detect meaningful training related responses in a variety of health, physical fitness and functional performance outcomes and ensure no statistically significant between-group differences in outcome scores at baseline. These future studies should also be sufficiently powered to detect differences in training response as a function of interactions between different polymorphisms of multiple genes that may play a role in modulating the exercise response.

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